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HF Vertical Antennas
A vertical antenna is useful for "DX" work because the angle of radiation is typically very low - This means that your signal travels at a low angle outwards (ie: 20 degrees or less), bounces off the ionosphere and comes back down to earth again. With each "hop", the signal gets weaker. The reason why you want the least angle possible is because the signal travels further before bouncing, and if the angle is too high it won't bounce at all and carry-on into space!
The illustration below shows how the different angles react (and travel). The high-angled red signal only travels a short distance, while the purple and brown ones have a lower angle and thus travel further. With each "hop" the signal gets weaker so you want the least number of hops to increase your signal range - A signal that only takes 5 hops to reach it's destination will be stronger than had it took 10 hops. As you may remember from your licence studies, the gap in-between is known as the "skip-zone". This is why a UK station will easily hear Italians and Russians on 20m but probably not hear a station 50-100 miles away.
A simple example showing the
different angles of 3 signals
Radials do not produce a low-angle of radiation - They simply help the aerial operate efficiently, eg: the more efficient an aerial is, the more RF is radiated. This article, The Mystery of Radials is good reading. Vertical antennas tend to be noisy - A general rule-of-thumb is that if a vertical isn't "lively" then it's not working!
The image below shows a typical ground-mounted vertical arrangement. This comprises of a 2" pole pushed into the ground with about 12" above ground. A pair of U-bolts allows a mounting plate to be attached, with the vertical mounted on 2 insulated brackets. Telescoping sections of aluminium pole form the aerial allowing it to be retracted when not in use, easily stored, or transported for /P events.
One of the good things about this kind of base is that it can be used for all sorts of aerial experiments: If you need to test a yagi or put an aerial up above ground, you can add a joining-sleeve and longer pole. A copper earth-stake is pushed into the ground giving you the additional option of a proper earth connection. Adding a matching-coil to the mounting plate is also possible if you want to create a 5/8th wave on the higher bands. Why would you want to do this? Gain, is the answer! A 5/8th wave on 17m is possible using my 40m vertical, and the coil is easily made - just experiment! You also get a lower angle of radiation than the 1/4wave and is preferable for DX'ing. Details on construction and tuning will follow.
I can thoroughly recommend Aerial Parts of Colchester for building this sort of antenna, either as a complete solution or for the individual parts. John G4ZTR offers a range of aluminium tube, fixings and even Alimast sections for making a tower. John's off-the-peg vertical is advertised as a 30m-10m 1/4wave but he is happy to supply a longer 40m version if requested. Guying is required if the aerial is fully extended.
My version is around 1.85m retracted, so quite portable as well as easily stored during periods of inactivity. Of course, you can "cheat" and run it via your ATU but if you've done your workings-out beforehand, you can mark the sections with the positions for each band and simply loosen the hose-clips to re-tune the aerial. During a recent IARU HF Contest, with the aerial raised for 40m, I worked: LU5FC, CR3T, XE1KK and LR5H on 15m (odd harmonic) with signals 1-2 S-points over my wire dipole. Mine is fed with a short run of RG-213 coax into a home-brew Bottle-Choke (20ft of RG58).
A simple SWR meter or antenna analyzer, such as the MFJ-259 or Youkits FG-01 are really useful devices when experimenting with aerials - They give you an instant idea of how the aerial is performing and make tuning a resonant antenna a quick and simple process. Here's a map of stations worked using the HF Vertical. Typically, it's extended to 10m for use on 80m/40m/15m but on 30m/20m/17m it has been used as a 1/4wave. On 12m+10m it has also been used on an odd-harmonic. You could turn it into a 5/8th-wave for perhaps 10/12m by adding a matching-coil to the base.
The antenna and base-plate cost £71 for the 40m custom version, or £57 for the standard 30m-10m (7.9m) offering. The bottom of the mounting plate has 4 holes drilled for attaching your radials via a solder-tag. You'd obviously need to cut these and adjust the aerial height for each band and mark the sections to indicate the positions of each band. When it's up for 40m, it also kicks-ass on 15m as an odd-harmonic. Where I've used this on 15m, you'll see 40m Vertical on my Logbook. You can click here for a GoogleMaps plot of stations worked using this dipole
Update : 10th Nov 2012 : John G4ZTR has recently revised the design of the vertical, making the base-section a slightly larger size and of an increased wall thickness. The cost has increased a bit but if you are after a rugged 40m Vertical, John can probably help.